The present invention relates to blade arrangements and more particularly to blade arrangements of a so-called blisk nature utilised in gas turbine engines.
Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts 26, 28, 30.
In view of the above it will be appreciated that blade assemblies comprise a number of blades secured around generally a rotatable hub or rotor. Traditionally, these blades have been secured through appropriate blade roots possibly of a fir tree nature. Such connections for the blades can add significantly to weight and complexity of formation. In such circumstances more recently alternative blade constructions have been proposed and utilised. In particular with regard to compressor stages and with respect to small turbines it is possible to create a blisk format. In such circumstances generally a disc of material is utilised to act as a rotor upon which blades are secured through an appropriate welding technique. The blades are either cast with the disc as one piece or as indicated bonded by an appropriate friction welding or similar joining process. In such circumstances the blades are simply secured by an appropriate integral or bonded weld joint to the disc without the necessity for blade roots etc to secure the blades to the rotor. Thus, the number of machining and other processes is reduced as well as the weight of the assembly.
When designing a blade arrangement it is important to consider vibration, to protect the disc and in particular the joint between the blade and the disc from excessive heating and also to define an appropriate aerodynamic profile for operational purposes. With regard to vibration previous arrangements typically have not considered vibration as an issue as the resonant frequency can be designed out of the running range of the blade. Protection of a disc rim from annulus gas is also an issue as indicated but generally the disc material is capable of taking the annulus gas temperature as it is formed from the same material as the blade. Furthermore, it is possible to profile end walls in order to improve aerodynamic performance but not without difficulty. Previous arrangements have limited design options.